Calibration (GCAL)

A Gemini Facility Calibration Unit (GCAL) is available on each
Gemini telescope, providing continuum and line emission light sources
for wavelength and flat-field calibrations. Each GCAL is located
permanently on one port of the instrument support
structure and light is directed into an instrument by the science fold
mirror. The GCAL optics simulate the f/16 telescope beam, illuminating the instrument pupil
in the same way
as light from an astronomical source. Instruments within the wavelength
range 0.3µm - 5mm can
be calibrated; mid-IR instruments are calibrated by other means.

GCAL is operated via flat and arc observations defined in the OT as part of each program's Phase II definition. Proper selection of lamps, ND filters and exposure times depends on the instrument being used and more information is available on the individual instrument web pages. The available line and continuum calibration lamps are shown in the table below.

Notes: In March 2005 Krypton lamp replaced with Xenon for higher line density. GS ThAr lamp was removed for 100W QTH required for PHOENIX (visiting instrument) but the lamp is still in inventory.

Filters: A single eight-position filter wheel with ND and CB filters is available in the GCAL unit for flux attenuation and for flux gradients reduction of the QH lamp. Filters include ND1.0, ND2.0, ND3.0, ND4.0, ND4-5, GMOS CB and NIR CB.

Diffusers: Visible and IR diffusers are provided. Note the IR diffuser has two emission lines that may affect flats, as was observed on GMOS.

Intensity stability: The intensity stability of the Grey Body source reflects the stability of the power supply and is expected to be ±1%. The intensity stability of the QH lamp (from the manufacturers’ specification) is ± 0.4% rms.

Output beam stability: The original instrument requirement states that the output beam should be flat with a monotonic roll-off in intensity of <10% to the edge of the 7' diameter field, and that variations over the 3’ field should be 1%. To achieve a flatter beam than this requires calibration of the GCAL against the sky with each of the instruments.